Intrinsically safe electrical control system



May 20, 1969 c. F. MEYER ETAL I INTRINSICALLY SAFE ELECTRICAL CONTROLSYSTEM Filed Dec. 15, 1966 INVENTOR.

CHARLES F. MEYER ROBERT A.WIECZOREK E/M J United States Patent 3,445,679INTRINSICALLY SAFE ELECTRICAL CONTROL SYSTEM Charles F. Meyer,Wauwatosa, and Robert A. Wieczorek, Milwaukee, Wis., assignors to SquareD Company, Park Ridge, [1]., a corporation of Michigan Filed Dec. 15,1966, Ser. No. 602,089 Int. Cl. H01h 9/30 US. Cl. 307136 9- ClaimsABSTRACT OF THE DISCLOSURE The disclosure pertains to an intrinsicallysafe circuit wherein the transmission of electrical energy to aswitching device located in a hazardous area is limited to a valueinsufficient to cause an explosion in the hazardous area upon any typeof malfunction of the circuit. Malfunction of the circuit from any causeis prevented by spirally wound resistors, a grounded shield transformer,and a light coupling between the circuit in the hazardous area and theremainder of the circuit located in the nonhazardous area through aphotocell and a lamp to prevent transmission of dangerous electricalenergy to the switching device in the hazardous area from the switchingcircuits in the nonhazardous area.

The present invention relates to electrical control systems and is moreparticularly concerned with an intrinsically safe electrical switchingsystem for use in connection with areas wherein explosive atmospheresare present.

The requirements of an intrinsically safe control system are generallyexpressed in the National Electric Code as follows:

Equipment and associated wiring approved as intrinsically safe may beinstalled in any hazardous location for which it is approved and theprovisions pertaining to conventional explosion-proofing procedures neednot apply to such installation. Intrinsically safe equipment and wiringis incapable of releasing suflicient electrical energy under normal orabnormal conditions to cause ignition of a specific hazardousatmospheric mixture. Abnormal conditions will include accidental damageto any part of the equipment or wiring, insulation or other failure ofelectrical components, application of over-voltage, adjustment andmaintenance operations, and other similar conditions.

It is an object of the present invention to provide an improvedacceptable intrinsically safe control system.

Another object of the present invention is to provide a control systemwherein an intrinsically safe circuit in a hazardous area iselectrically isolated from an output circuit in a nonhazardous area by alight emitting and a light sensitive element.

A further object is to provide an intrinsically safe switching systemfor controlling the operation of a switching device located in anonhazardous area from a control station located in a hazardous areawhere explosive mixtures of gases or dust may be present to eliminatethe requirement that electrical control equipment in the hazardous areabe enclosed in an explosion-proof housing or be prevented from ignitingthe explosive mixtures, as by purging, sealing and the like, and tocouple the circuit in the hazardous area with a supply circuit for aswitching device in the nonhazardous area through a photo sensitivedevice to electrically isolate the circiut in the hazardous area fromthe supply circuit and to supply the circuit in the hazardous area withan intrinsically safe amount of energy through resistors of the spirallywound type from a secondary winding of a stepdown type transformer thathas a grounded shield interposed between the primary and secondarywindings to prevent current from a high energy supply to the primarywinding of the transformer from flowing in the circuit in the hazardousarea in event of a short between the primary and second ary windings ofthe transformer.

Further objects and features of the invention will be readily apparentto those skilled in the art from the specification and the appendeddrawing illustrating a preferred embodiment in which an intrinsicallysafe electrical control system incorporating the features of the presentinvention is diagrammatically shown.

In the drawing, a barrier wall 10 separates an area 12 wherein explosivegases or dust are present from an area 14, having a nonhazardousatmosphere. Included within the area 12 is a control station 16 andincluded within the area 14 is a control circuit indicated by a numeral18. The control circuit 18 has a pair of input terminals 20 and 22connected via a pair of leads 24 and 26 to a source 28 of alternatingcurrent located in the nonhazardous area 14. The control circuit 18 alsohas a pair of output terminals 30 and 32 arranged to supply an outputsignal through a lead 36- to an external load 38, not specificallyshown. The control circuit 18 is enclosed within a housing 40 that hasan internal barrier 42 physically dividing the housing 40 into twocompartments indicated by the numerals 44 and 46. The terminals 20, 22,30 and 32, and a G terminal, as well as a switching device shown as arelay 48, are positioned within the compartment 46. The relay 48 has anormally open contact 49 connected across the terminals 30 and 32 whichclose and complete a circuit to the load 38 upon energization of therelay 48. Three terminals 50, 51 and 52, located within the compartment44, are connectible by leads 54, 55 and 56 to a switching circuit 60within the control station. The leads 54, 55 and 56 each extend througha suitable atmospheric seal 58 in the barrier Wall 10 to the switchingcircuit 60 located within the control station 16. The atmospheric seal58 prevents passage of the explosive atmosphere in the hazardous area 12to the nonhazardous area 14. The switching circuit 60 is provided with aswitch 62 having normally open contacts and a switch 63 having normallyclosed contacts. The switches 62 and 63 may be of the standard pushbutton variety, the contacts of a conventional limit switch, or thecontacts of a metering device or the like, and the control station 16may or may not have an explosion proof enclosure, as desired. The leads54, 55 and 56 are not required to have any type of explosion proofcovering and may extend a considerable distance from the control station16 to the control circuit 18. The barrier 42, the control station 16, aswell as the enclosure 40, are all illustrated by broken lines. Thebarrier 42 is positioned within the enclosure 40 to have a top edgeengageable with a bottom surface of a cover, not shown, for theenclosure to physically separate the compartments 44 and 46 and preventthe cover from being secured to the enclosure in event the leads 54, 55and 56 are attached to any one of the terminals 20, 22, 30 or 32 and/orthe leads 24, 26 and 36 are secured to the terminals 50*, 51 and 52. Inthe event that accidentally during installation any one of the leads areincorrectly thus positioned, the cover will be prevented from beingsecured to the enclosure by the leads which extend over the top edge ofthe barrier 42. Thus the control circuit 18 will be required to beproperly connected before the circuit is placed in operation.

Included within the compartment 44 is a step-down type transformer 61having a primary winding 64, a secondary winding 66, and an iron core68. The primary winding 64 and the secondary winding 66 are each woundupon the core 68 and are electrically separated by a metallic shield 70having a connection with a ground G. The primary winding 64 is connectedto the terminals and 22 to be supplied with alternating current from thesource 28.

Also included within the compartment 44 is a full wave bridge rectifier72, a capacitor 74, a resistor 76, a Zener diode 78, a light emittingelement shown as a lamp 82, and a light sensitive element 84, such as anonpolarized semiconductor whose resistance decreases when subjected tolight, a resistor 88, an electronic switch such as a silicon controlledrectifier 90 having an anode electrode 91, a cathode electrode 92 and agate electrode 93, and a firing and filter circuit for the rectifiergate circuit including the resistor 95 and a capacitor 94.

The rectifier 72 has a pair of input terminals 96 and 97 connected toreceive the output of the secondary winding 66 and diodes polarized toprovide a pair of output terminals 98 and 99 with full wave rectifiedcurrent making the terminal 98 positive in polarity relative to theterminal 99. The capacitor 74 has one side connected to the terminal 98and another side connected through a junction 100 to the terminal '99.The Zener diode 78 has an anode connected to the junction 100 and acathode connected through a junction 102 to the terminal 50. Theresistor 76 is connected between the terminal 98- and the junction 102in parallel with the capacitor 74 and the Zener diode 78.

The controlled rectified 90 has its anode 91 connected to the terminal52, its cathode 92 connected through the lamp 82 to the terminal 99 andits gate electrode 93 connected through the resistor 88 to the terminal51. Connected in parallel between the cathode 92 and the gate electrode93 are the resistor 95 and the capacitor 94. The light sensitive element84, which is physically located to receive light from the lamp 82, isconnected in series circuit with the operating coil of the relay 48across the alternating current supply 28. Included in the switchingcircuit 60 is a junction 103 which is directly connected by the lead 56to the terminal 52. Additionally, the junction 103 is connected throughthe closed contacts of the switch 63 and the lead 54 to the terminal 50and is connectible through the contacts of the switch 62, when theswitch 62 is operated, and the lead 55 to the terminal 51.

The operation of the circuit shown in the drawing is as follows. Duringstandby conditions the switch 62 is in a circuit opening condition andthe switch 63 is in a circuit closing condition. The voltage of thesource 28, as reduced by the primary 64 and the secondary 66 windings ofthe transformer 61, rectified by the rectifier 72, and filtered by thecapacitor 74, appears as a full wave rectified filtered voltage betweenthe terminals 98 and 99. The voltage between the terminals 102 and 99 islimited by the Zener diode 78. In the event the voltage between theterminals 102 and 99 exceeds the break-down characteristics of the diode78, the diode 78 will conduct, in the reverse direction, causing currentflow from the terminal 98, through a circuit which includes the resistor76, the junction 102, the diode 78 and the junction 100 to the terminal99. Also during standby conditions, the controlled rectifier 90 isnonconducting and the lamp 232 extinguished. Thus the light sensitiveelement 84 will be de-energized and have a high resistance causing therelay 48 to be de-energized so its contact 49 interrupts the circuit tothe load 38.

When the switch 62 is operated and its contacts are moved to a circuitclosing position, a firing circuit for the controlled rectifier 90 iscompleted from the terminal 98 through the resistor 76, the junction102, the terminal 50, the lead 54, the closed contacts of the switch 63,the closed contacts of the switch 62, the lead 55, the terminal 51, theresistor 88, the gate to cathode electrodes 93 and 92 of the controlledrectifier 90 and the lamp 82 to the terminal 99. The gate to cathodecurrent through the .4 rectifier switches the rectifier 90 to aconductive state that is maintained as direct current flows from theterminal 98 through the resistor 76, the junction 102, the terminal 50,the lead 54, the closed contacts of switch 63, the lead 56, the terminal52 and the anode 91 to cathode 92 of the rectifier 90 and the lamp 82 tothe terminal 99. The anode to cathode current flow through the rectifier90 causes: the lamp 82 to be illuminated, the resistance of the lightsensitive element 84 to markedly decrease and the relay 48 to beenergized so that the contact 49 closes and completes a circuit throughthe load 38. The conductive state of the rectifier 90 continues as longas the switch 63 is closed, permitting the switch 62 to be opened, afterits initial operation, without de-energizing the circuit which includesthe lamp 82. The de-energization of the load 38 is achieved when thecontrolled rectifier is switched to a nonconductive state by opening theswitch contacts 63 to interrupt the anode circuit of the controlledrectifier 90. When the lamp 82 is de-energized as the rectifier 90becomes nonconductive, the resistance of the light sensitive element 84again increases to a value which causes the relay 48 to be deenergizedand the contacts 49 to interrupt the circuit to the load 38.

The intrinsically safe features of the circuit are basically achieved bythe combination of the spirally wound resistors, the grounded shieldtransformer and the isolation provided by the lamp-photocell in thecircuit. The resistors 76, 88 and 95 each are spirally wound with a wirehaving a positive temperature coefficient without crossovers and sealedin a crack-free, fireproof ceramic case. Thus upon failure, theresistors will fail to an open circuit state. The transformer 62, byhaving a grounded shield 70 between the primary winding 64 and thesecondary winding 66, is protected against short circuits between theprimary and secondary winding and prevents possible high energy levelcurrents from the supply 28 from flowing in the switching circuit 60.The isolation provided by the lamp 82 and the photocell 84 positivelyprevents transmission of dangerous currents from the relay 48 circuit tothe switching circuit 60. Finally in the preferred embodiment, thecomponents in the compartment 44 are protected from physical damage byencapsulating the same in a suitable epoxy resin, not shown.

What is claimed is:

1. An intrinsically safe switching system for controlling the operationof a switching device located in a nonhazardous area from a controlstation in a hazardous area where an explosive atmospheric mixture maybe present, comprising:

(a) an actuable switching circuit in the hazardous area, said switchingcircuit having at least a portion that is not sealed or provided withexplosion proof enclosing means or the like,

(b) a source of alternating current in the nonhazardous area,

(c) a control circuit in the nonhazardous area, said control circuithaving a connection through a first circuit with the switching circuitand a connection through a second circuit With the switching device andisolated from the switching circuit in the hazardous area by a hermeticseal which prevents passage of the explosive mixture between thehazardous and the nonhazardous areas.

(d) said control circuit including: means in the first circuit forsupplying a small and intrinsically safe amount of electrical energythrough the switching circuit insufiicient to ignite the explosivemixture in the hazardous area when the switching circuit is actuated,and means including a light emitting element in the first circuitproviding an output signal in response to the flow of electrical energyin the switching circuit and a light sensitive element in the secondcircuit for receiving the output signal from the light emitting elementand causing the switching device to be energized from the source inresponse to the output signal and electrically isolating the secondcircuit from the first circuit.

2. The combination as recited in claim 1 including a transformer havinga primary winding connected across the source, a secondary windingconnected to supply the first circuit and the switching circuit with theintrinsically safe energy and the second circuit with energizing currentand a ground shield interposed between the primary and secondarywindings for preventing unsafe electrical energy from flowing in theswitching circuit in event of a short circuit between the primary andthe secondary windings.

3. The combination as recited in claim 1 including at least one spirallywound resistor connected in a series circuit between the source and theswitching circuit for limiting current flow in the switching circuit.

4. The combination as recited in claim 2 including at least one spirallywound resistor connected in a series circuit between the source and theswitching circuit for limiting current flow in the switching circuit.

5. The combination as recited in claim 2 including a full wave rectifierhaving a pair of input terminals connected across the secondary windingand a pair of output terminals connected to supply the switching circuitand the first circuit with the electrical energy.

6. The combination as recited in claim 5 including a spirally woundresistor connected in series circuit between the switching circuit andthe output terminals of the rectifier.

7. The combination as recited in claim 5 including a silicon controlledrectifier having an anode and a cathode connected in a series circuitwith the light emitting device and a gate and the cathode connected in aseries circuit with the switching circuit for energizing the lightemitting device with direct current when the switching means isactuated.

8. The combination as recited in claim 7 including a switch havingnormally open contacts connected in the switching circuit in series withthe gate of the controlled rectifier.

9. The combination as recited in claim 7 including a switch havingnormally closed contacts connected in the switching circuit in serieswith the anode of the controlled rectifier.

References Cited UNITED STATES PATENTS 7/1965 Elliot 307136 3/1967Elliot 307l36 US. Cl. X.R. 317123

